The present invention relates to mechanical kinetic vacuum pumps. It finds particular applications to vacuum pumps in which the rotor and stator are connected by a shrink-fit or a screw-fit.
By definition gaseous ring vacuum pumps, turbo vacuum pumps (axial, radial) and molecular/turbomolecular pumps belong to the class of mechanical kinetic vacuum pumps. They are capable of mechanically transporting within the molecular flow range (pressures below 10−3 mbar) the gas particles which are to be pumped. Moreover, molecular pumps are also capable of pumping gases within the Knudsen flow range (10−3 to 1 mbar). Presently employed mechanical kinetic vacuum pumps frequently offer a turbomolecular pumping stage and a downstream molecular pumping stage (compound or hybrid pump), since such pumps are capable of compressing gases up in to the viscous flow range.
Pumps of the kind affected here, in particular turbomolecular vacuum pumps are operated at high rotational speeds up to 100,000 rpm. This requires a firm and tight joint between rotor and shaft which meets the requirements regarding rotor dynamics when passing through critical speeds. The joint commonly is provided by a shrink- or screw-fit. The shrink-fit joint is provided by joining the warm rotor and the cooled shaft, in that the shaft is introduced into a bore in the rotor. Generally steel is employed as the material for the shaft, since steel has a relatively high modulus of elasticity. For reasons of rotor dynamics a lighter material, preferably aluminium, is employed as the rotor material. Here aluminium alloys produced by melt metallurgy, e.g., casting, are well proven. However, in the instance of the material pair of steel/aluminium it is difficult to implement a joint between rotor and shaft which is free of backlash and settling since the coefficients of expansion of steel (about 11×10−6/K) and aluminium (about 22×10−6/K) differ.
From U.S. application Ser. No. 09/937,876, it is known to achieve freedom from backlash and settling at the joint between rotor and stator by providing reinforcement rings preventing an expansion of the aluminium rotor which would give rise to backlash. These measures are involved engineering-wise.